Cryogenic on-chip microwave filter for quantum devices

ABSTRACT

An on-chip microwave filter circuit includes a substrate formed of a first material that exhibits at least a threshold level of thermal conductivity, wherein the threshold level of thermal conductivity is achieved at a cryogenic temperature range in which a quantum computing circuit operates. The filter circuit further includes a dispersive component configured to filter a plurality of frequencies in an input signal, the dispersive component including a first transmission line disposed on the substrate, the first transmission line being formed of a second material that exhibits at least a second threshold level of thermal conductivity, wherein the second threshold level of thermal conductivity is achieved at a cryogenic temperature range in which a quantum computing circuit operates. The dispersive component further includes a second transmission line disposed on the substrate, the second transmission line being formed of the second material.

TECHNICAL FIELD

The present invention relates generally to a device, a circuit designmethod, and a circuit construction system for a microwave filter usablewith superconducting qubits in quantum computing. More particularly, thepresent invention relates to a device, method, and system for acryogenic on-chip microwave filter for quantum devices.

BACKGROUND

Hereinafter, a “Q” prefix in a word of phrase is indicative of areference of that word or phrase in a quantum computing context unlessexpressly distinguished where used.

Molecules and subatomic particles follow the laws of quantum mechanics,a branch of physics that explores how the physical world works at themost fundamental levels. At this level, particles behave in strangeways, taking on more than one state at the same time, and interactingwith other particles that are very far away. Quantum computing harnessesthese quantum phenomena to process information.

The computers we use today are known as classical computers (alsoreferred to herein as “conventional” computers or conventional nodes, or“CN”). A conventional computer uses a conventional processor fabricatedusing semiconductor materials and technology, a semiconductor memory,and a magnetic or solid-state storage device, in what is known as a VonNeumann architecture. Particularly, the processors in conventionalcomputers are binary processors, i.e., operating on binary datarepresented in 1 and 0.

A quantum processor (q-processor) uses the odd nature of entangled qubitdevices (compactly referred to herein as “qubit,” plural “qubits”) toperform computational tasks. In the particular realms where quantummechanics operates, particles of matter can exist in multiplestates—such as an “on” state, an “off” state, and both “on” and “off”states simultaneously. Where binary computing using semiconductorprocessors is limited to using just the on and off states (equivalent to1 and 0 in binary code), a quantum processor harnesses these quantumstates of matter to output signals that are usable in data computing.

Conventional computers encode information in bits. Each bit can take thevalue of 1 or 0. These 1s and 0s act as on/off switches that ultimatelydrive computer functions. Quantum computers, on the other hand, arebased on qubits, which operate according to two key principles ofquantum physics: superposition and entanglement. Superposition meansthat each qubit can represent both a 1 and a 0 at the same time.Entanglement means that qubits in a superposition can be correlated witheach other in a non-classical way; that is, the state of one (whether itis a 1 or a 0 or both) can depend on the state of another, and thatthere is more information that can be ascertained about the two qubitswhen they are entangled than when they are treated individually.

Using these two principles, qubits operate as more sophisticatedprocessors of information, enabling quantum computers to function inways that allow them to solve difficult problems that are intractableusing conventional computers. IBM has successfully constructed anddemonstrated the operability of a quantum processor usingsuperconducting qubits (IBM is a registered trademark of InternationalBusiness Machines corporation in the United States and in othercountries.)

A superconducting qubit includes a Josephson junction. A Josephsonjunction is formed by separating two thin-film superconducting metallayers by a non-superconducting material. When the metal in thesuperconducting layers is caused to become superconducting—e.g. byreducing the temperature of the metal to a specified cryogenictemperature —pairs of electrons can tunnel from one superconductinglayer through the non-superconducting layer to the other superconductinglayer. In a qubit, the Josephson junction—which functions as adispersive nonlinear inductor—is electrically coupled in parallel withone or more capacitive devices forming a nonlinear microwave oscillator.The oscillator has a resonance/transition frequency determined by thevalue of the inductance and the capacitance in the qubit circuit. Anyreference to the term “qubit” is a reference to a superconducting qubitcircuitry that employs a Josephson junction, unless expresslydistinguished where used.

The information processed by qubits is carried or transmitted in theform of microwave signals/photons in the range of microwave frequencies.The microwave signals are captured, processed, and analyzed to decipherthe quantum information encoded therein. A readout circuit is a circuitcoupled with the qubit to capture, read, and measure the quantum stateof the qubit. An output of the readout circuit is information usable bya q-processor to perform computations.

A superconducting qubit has two quantum states —|0> and |1>. These twostates may be two energy states of atoms, for example, the ground (|g>)and first excited state (|e>) of a superconducting artificial atom(superconducting qubit). Other examples include spin-up and spin-down ofthe nuclear or electronic spins, two positions of a crystalline defect,and two states of a quantum dot. Since the system is of a quantumnature, any combination of the two states are allowed and valid.

For quantum computing using qubits to be reliable, quantum circuits(q-circuits), e.g., the qubits themselves, the readout circuitryassociated with the qubits, and other parts of the quantum processor,must not alter the energy states of the qubit, such as by injecting ordissipating energy in any significant manner, or influence the relativephase between the |0> and |1> states of the qubit. This operationalconstraint on any circuit that operates with quantum informationnecessitates special considerations in fabricating semiconductor andsuperconducting structures that are used in such circuits.

The presently available superconducting quantum circuits are formedusing materials that become superconducting at cryogenically lowtemperatures, e.g., at about 10-100 millikelvin (mK), or about 4 K. Theelectronic circuits that are used to control, operate, and measure thequantum circuits are usually located outside the dilution fridge thathouses the superconducting quantum circuit. The temperature outside thefridge is usually about 300 K (room temperature).

The presently available superconducting quantum circuits usually operatein the microwave frequency range. Microwave signals/pulses are used toinitialize, manipulate, control, and measure the superconducting qubitswithin the superconducting q-circuits. To communicate these microwavesignals between the external electronic circuits outside the fridge andthe superconducting quantum circuits inside the fridge, microwavetransmission lines are used inside the dilution fridge. Coaxial linesare one example of transmission lines that can carry these microwavesignals.

The presently available dilution fridges are cryogenic apparatus whichcan be used to cool down samples/devices to millikelvin temperatures.However, the transition from room temperature to millikelvintemperatures inside the fridge is not sudden or abrupt. To facilitatethe temperature transition and the cooling operation, the dilutionfridge consists of multiple thermally-isolated stages (compactlyreferred to herein as “stage”, plural “stages”) held at differentambient temperatures. For example, common commercial dilution fridgeshave 5 temperature-stages inside the fridge 40 K, 4 K, 0.7 K, 0.1 K,0.01 K (also known as the base stage). To simplify the discussion, wefocus below on the input lines inside the fridge. To maintain thetemperature difference between the different stages inside the fridgeand to protect the quantum circuits from noise coming down the inputlines, which originates from room-temperature electronics or blackbodyradiation of higher stages or other sources of electromagnetic noise, itis common practice to use lossy transmission lines to connect betweentwo consecutive stages and to incorporate resistive attenuators andfilters in the path of these lines at the different stages. In general,components serve multiple purposes: they filter/reduce the noise comingdown these input lines, they filter/reduce microwave signals propagatingin the lines, they provide thermal isolation between the stages, andthey cool down the microwave signals propagating through them.

A signal propagating on a line between stages can contain hot electrons,electrons containing more energy as a result of being located outsidethe fridges at room temperature. Hot electrons can bring thermal noiseinto the stages. This noise can be in the infrared spectrum.

A signal propagating on a line passing through a stage can containnoise, especially electromagnetic noise. This noise can be in themicrowave frequency spectrum or infrared spectrum. For the reasonsdescribed herein, electronic, microwave and infrared noise areundesirable when the lines and signals relate to quantum computing usingq-circuits.

Filtration of a signal is the process of reducing the power of thesignal at a particular frequency or frequency-range. A filter is anelectronic circuit with two ports that is configured to filter an inputsignal/noise at a particular frequency or frequency-range.

A dispersive filter filters the transmitted signal/noise through its twoports by reflecting a portion of its energy/power off the port itentered through.

The illustrative embodiments recognize that commercially availablestandard microwave filters are not designed to operate in the cryogenictemperature range, lower than about 77 K down to about 0.01 K. Forexample, in most cases, the transmission line materials used for thesefilters have far from ideal thermal conductance. The illustrativeembodiments also recognize that physical connections between a filtersubstrate, signal lines, and filter housing promote removal of thermalenergy in the signal lines. Furthermore, the illustrative embodimentsrecognize certain disadvantages with the presently available microwavefilters. For example, in most cases, the presently available microwavefilters are formed such that a metallic case enclosing the substrate isstainless steel, which has poor thermal conductance. Additionally, theconnectors located on either port of the microwave filter are notstandardized from one microwave filter to the next, which increasesreflections of signals in the line, which, in turn, can causedistortions in the microwave pulses/signals, crosstalk, and ripples inthe measured signals. In addition, the substrate materials used forthese filters have far from ideal thermal conductance.

SUMMARY

The illustrative embodiments provide an electronic attenuating device,and a method and system of fabrication therefore. In one embodiment, anon-chip microwave filter circuit includes a substrate formed of a firstmaterial that exhibits at least a threshold level of thermalconductivity, wherein the threshold level of thermal conductivity isachieved at a cryogenic temperature range in which a quantum computingcircuit operates. The filter circuit of the embodiment further includesa dispersive component configured to filter a plurality of frequenciesin an input signal, the dispersive component including a firsttransmission line disposed on the substrate, the first transmission linebeing formed of a second material that exhibits at least a secondthreshold level of thermal conductivity, wherein the second thresholdlevel of thermal conductivity is achieved at a cryogenic temperaturerange in which a quantum computing circuit operates. The dispersivecomponent of the filter circuit of the embodiment further includes asecond transmission line disposed on the substrate, the secondtransmission line being formed of the second material.

In another embodiment, an on-chip microwave filter circuit includes aconnector coupled to the first transmission line, the connector beingformed of a third material that exhibits at least a third thresholdlevel of thermal conductivity, wherein the third threshold level ofthermal conductivity is achieved at a cryogenic temperature range inwhich a quantum computing circuit operates.

In another embodiment, the filter circuit includes a second connectorcoupled to the second transmission line, the second connector beingformed of the third material.

In another embodiment, the filter circuit includes a housing coupled tothe substrate, the housing being formed of a fourth material thatexhibits at least a fourth threshold level of thermal conductivity,wherein the fourth threshold level of thermal conductivity is achievedat a cryogenic temperature range in which a quantum computing circuitoperates.

In another embodiment, the dispersive component of the filter circuitincludes a third transmission line disposed on the substrate, the thirdtransmission line being formed of the second material.

In another embodiment, the third transmission line is disposed on thesubstrate between the first transmission line and the secondtransmission line.

In another embodiment, the third transmission line is spaced apart onthe substrate from the first transmission line.

In another embodiment, the first transmission line has a thickness ofabout 10 nm to 1000 nm.

In another embodiment, the first transmission line has a width of about0.5 mm.

An embodiment includes a fabrication method for fabricating the on-chipmicrowave filter circuit.

An embodiment includes a system for fabricating the on-chip microwavefilter circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives and advantages thereof, willbest be understood by reference to the following detailed description ofthe illustrative embodiments when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 depicts a block diagram of an example configuration of an inputline conditioning for quantum computing devices in accordance with anillustrative embodiment;

FIG. 2A depicts an example configuration of an on-chip microwave filterin accordance with an illustrative embodiment;

FIG. 2B depicts an example circuit implementing an on-chip microwavefilter in accordance with an illustrative embodiment;

FIG. 3 depicts an example on-chip microwave filter 300 in accordancewith an illustrative embodiment;

FIG. 4 depicts microwave simulation results of the scattering parametersof an on-chip microwave filter example; and

FIG. 5 depicts a flowchart of an example method 500 for assembling anon-chip microwave filter.

DETAILED DESCRIPTION

The illustrative embodiments used to describe the invention generallyaddress and solve the above-described needs for attenuating certainmicrowave signals mainly in the input lines connecting to q-circuits.The illustrative embodiments provide on-chip microwave filters forquantum circuits, which address the above-described need or problem.

An operation described herein as occurring with respect to a frequencyof frequencies should be interpreted as occurring with respect to asignal of that frequency or frequencies. All references to a “signal”are references to a microwave signal unless expressly distinguishedwhere used.

The illustrative embodiments recognize that performance of anysuperconducting based quantum architecture is heavily dependent on thequality of the superconducting qubits themselves which can be directlycharacterized by the measuring coherence times. These times stronglydepend on the performance of microwave hardware—not only the qubitsthemselves but also the filters used in the microwave lines and thehousing for the filters—at temperature range where quantum computingcircuits operate. In order to increase the coherence times, thus improvethe functionality of the quantum processors, all the microwavecomponents, control lines, components, and packaging are thermalized toa quantum computing-acceptable level of thermalization.

The illustrative embodiments allow for better thermalization of themicrowave components and cleaner microwave signal throughput, byutilizing a filter design and high thermal conductivity materials in thefabrication of the filter and the housing, as described herein. Theillustrative embodiments recognize that physical connections between afilter substrate, housing, and signal lines promote removal of thermalenergy from the signal lines.

An embodiment provides a configuration of an on-chip microwave filterthat operates at cryogenic temperatures. Another embodiment provides adesign/construction method for the on-chip microwave filter, such thatthe method can be implemented as a software application. The applicationimplementing a design/construction method embodiment can be configuredto operate in conjunction with an existing circuit manufacturingsystem—such as a circuit assembly system.

For the clarity of the description, and without implying any limitationthereto, the illustrative embodiments are described using some exampleconfigurations. From this disclosure, those of ordinary skill in the artwill be able to conceive many alterations, adaptations, andmodifications of a described configuration for achieving a describedpurpose, and the same are contemplated within the scope of theillustrative embodiments.

Furthermore, simplified diagrams of the example resistors, inductors,capacitors, and other circuit components are used in the figures and theillustrative embodiments. In an actual circuit, additional structures orcomponent that are not shown or described herein, or structures orcomponents different from those shown but for a similar function asdescribed herein may be present without departing the scope of theillustrative embodiments.

Furthermore, the illustrative embodiments are described with respect tospecific actual or hypothetical components only as examples. The stepsdescribed by the various illustrative embodiments can be adapted forfabricating a circuit using a variety of components that can be purposedor repurposed to provide a described function within an on-chipmicrowave filter, and such adaptations are contemplated within the scopeof the illustrative embodiments.

The illustrative embodiments are described with respect to certain typesof materials, electrical properties, steps, numerosity, frequencies,circuits, components, and applications only as examples. Any specificmanifestations of these and other similar artifacts are not intended tobe limiting to the invention. Any suitable manifestation of these andother similar artifacts can be selected within the scope of theillustrative embodiments.

The examples in this disclosure are used only for the clarity of thedescription and are not limiting to the illustrative embodiments. Anyadvantages listed herein are only examples and are not intended to belimiting to the illustrative embodiments. Additional or differentadvantages may be realized by specific illustrative embodiments.Furthermore, a particular illustrative embodiment may have some, all, ornone of the advantages listed above.

With reference to FIG. 1, this figure depicts a block diagram of anexample configuration of an input line conditioning for quantumcomputing devices in accordance with an illustrative embodiment.Configuration 100 comprises a set of one or more dilution fridge stages102, 104, . . . 106. Input line 108 connects an external circuit toq-circuit 110. Assuming that line 108 carries a microwave signal toq-circuit 110, signal S₁ is a signal which includes microwave noise thatis to be filtered along with the signal S₁. Signal S_(n) is the filteredsignal that reaches q-circuit 110.

One embodiment configures an on-chip microwave filter with some but notall of stages 102-106. Another embodiment configures an on-chipmicrowave filter with each of stages 102-106, as shown in FIG. 1. Forexample, on-chip microwave filter 112 is configured to operate withstage 102. On-chip microwave filter 112 receives input signal S₁ andreflected signal S_(R2) from subsequent stages in the series of stages.On-chip microwave filter 112 filters one frequency or frequency bandfrom the (S₁+S_(R2)) signal to produce signal S₂.

On-chip microwave filter 114 is configured to operate with stage 104.On-chip microwave filter 114 receives input signal S₂ and reflectedsignal S_(R3) from subsequent stages in the series of stages. On-chipmicrowave filter 114 attenuates a different frequency or frequency bandfrom the (S₂+S_(R3)) signal to produce signal S₃. Operating in thismanner, stage 116 (stage n) has on-chip microwave filter 116 configuredtherewith. On-chip microwave filter 116 receives input signal S_(n-1)(and possibly a reflected signal if q-circuit 110 is configured toreflect any signal frequencies, not shown) from previous stages in theseries of stages. On-chip microwave filter 116 filters a differentfrequency or frequency band from the (S_(n-1)+any reflected frequencies)signal to produce signal S_(n), which forms an input to q-circuit 110.

With reference to FIG. 2A, this figure depicts one example configurationof an on-chip microwave filter in accordance with an illustrativeembodiment. The example configuration in this figure of on-chipmicrowave filter 200 comprises bandpass filter 202. On-chip microwavefilter 200 can be implemented as a two-port integrated circuit. Bandpassfilter 202 is a dispersive filter to allow a frequency band that isbetween two threshold frequencies (and filters/blocks frequenciesoutside this pass band). According to one embodiment, a circuit assemblysystem forms the bandpass filter component 202 on a chip or printedcircuit board.

With reference to FIG. 2B, this figure depicts an example circuit 204implementing an on-chip microwave filter in accordance with anillustrative embodiment. Component 206 is a dispersive element whichimplements a bandpass filter to allow a frequency band that is betweentwo threshold frequencies (and filters/blocks frequencies outside thispass band).

Component 206 comprises a configuration of capacitive elements L3 and C3in parallel and coupled to ground, i.e., the external conductor of theof the on-chip microwave filter. L3-C3 couple to L1-C1 series and L2-C2series via an internal conductor of the on-chip microwave filter on theother side, as shown. Component 206 also comprises a configuration ofcapacitive elements L4 and C4 in parallel and coupled to ground, i.e.,the external conductor of the on-chip microwave filter. L4-C4 couple toL2-C2 series via an internal conductor of the on-chip microwave filteron the other side, as shown. The depiction of component 206 and elementsL1-L4 and C1-C4 are lumped realizations, i.e., a representation of aneffective function of component 206 as a bandpass filter in themicrowave frequency band. This example shows a simple one-unit-cell,bandpass filter. This design also covers cases in which the simplebandpass filter shown in FIG. 2B is replaced by amore sophisticatedbandpass filter that consists of several unit cells and whoseattenuation, transmission, bandwidth, cutoff frequency, and ripplescharacteristics are optimized further or differently.

In component 206, capacitive elements C1 and C2 on the internalconductor of the on-chip microwave filter serves as DC blocks, which canbe used to eliminate the formation of ground loops in the fridge. Suchground loops are undesirable as they can generate electronic noise.Inductive element L3 connected the center and external conductors of theon-chip microwave filter offers a path of negligible resistance betweenthe center conductor and the external conductor of the on-chip microwavefilter.

The lumped realization of component 206 is not intended to be limiting.From this disclosure, those of ordinary skill in the art will be able toconceive many other implementations for a depicted lumped realization,e.g., using additional or different elements to achieve a similarfunction of the lumped realization shown here, and such implementationsare contemplated within the scope of the illustrative embodiments.

With reference to FIG. 3, this figure depicts an example on-chipmicrowave filter 300 in accordance with an illustrative embodiment. Theexample on-chip microwave filter 300 comprises substrate 302, dispersivecomponent including transmission lines 304, 306, 308, connectors 310,312, and housing 314. Substrate 302 comprises a material with highthermal conductivity (above a threshold) in the cryogenic temperaturerange. In an embodiment, substrate 302 is formed using a material thatexhibits a Residual Resistance Ratio (RRR) of at least 100, and athermal conductivity of greater than a 1 W/(cm*K) at 4 Kelvin, thresholdlevel of thermal conductivity. RRR is the ratio of the resistivity of amaterial at room temperature and at 0 K. Because 0 K cannot be reachedin practice, an approximation at 4 K is used. For example, substrate 302may be formed using sapphire, silicon, quartz, gallium arsenide, fusedsilica, amorphous silicon, or diamond for operations in the temperaturerange of 77K to 0.01K. These examples of substrate materials are notintended to be limiting. From this disclosure, those of ordinary skillin the art will be able to conceive many other materials suitable forforming the substrate and the same are contemplated within the scope ofthe illustrative embodiments.

Transmission lines 304, 306, 308 comprise a material with high thermalconductivity (above a threshold) in the cryogenic temperature range. Inan embodiment, transmission lines are formed using a metal that exhibitsa RRR of at least 100, and a thermal conductivity of greater than a 1W/(cm*K) at 4 Kelvin, threshold level of thermal conductivity. Forexample, transmission lines may be formed from using gold, silver,copper, or aluminum. These examples of transmission line materials arenot intended to be limiting. From this disclosure, those of ordinaryskill in the art will be able to conceive many other materials suitablefor forming the substrate and the same are contemplated within the scopeof the illustrative embodiments. In an embodiment, transmission lines304, 306, 308 are thin film depositions on substrate 302. In anon-limiting embodiment, transmission lines have a thickness in a rangeof about 10 nm-1000 nm. Transmission lines 304, 306, 308 are depositeddirectly on substrate 302 in an embodiment. For example, transmissionlines may be deposited using any conventional physical or chemical thinfilm deposition process, such as thermal evaporation, chemical vapordeposition, and sputtering.

With reference to FIG. 3, this figure depicts three transmission lines304, 306, 308 in accordance with an illustrative embodiment.Transmission line 306 is disposed on the substrate between transmissionlines 304, 308. Transmission line 306 is spaced apart on the substratefrom each of transmission lines 304, 308. Three transmission lines aredepicted only as a non-limiting example. In some embodiments, on-chipmicrowave filter 300 includes two, four, five or more transmissionlines.

In an embodiment, transmission lines 304, 306, 308 are configured toattenuate a plurality of frequencies in an input signal. Transmissionlines 304, 306, 308 operate as a bandpass filter for the quantumcircuit. Alternate configurations of transmission lines can beconfigured to filter a second plurality of frequencies in an inputsignal. For example, alternate configurations can be configured tofilter an increased/decreased frequency bandwidth of a plurality offrequencies. For example, at least one of a thickness of at least onetransmission line, a number of transmission lines, a length of at leastone transmission line in the direction of a length of a connector, awidth of at least one transmission line, and a material of at least onetransmission line can be altered to filter different pluralities offrequencies of an input signal and different bandwidths of a pluralityof frequencies of an input signal. In an embodiment, the length of atleast one transmission line is increased to filter a second plurality offrequencies lower than a first plurality of frequencies.

Connectors 310, 312 couple to transmission lines 304, 308 at oppositeends of substrate 302, respectively. For example, connectors 310, 312may be soldered to transmission lines 304, 308, respectively. In anon-limiting embodiment, transmission lines 304, 306, 308 include aminimum width of about 0.5 mm for coupling to the connectors 306, 308.Connectors 310, 312 are specifically configured for usability withsignals in the microwave frequency range. In accordance with anillustrative embodiment, connectors 310, 312 are selected to be of thesame type and gender. For example, connectors 310, 312 can both beSubMiniature version A (SMA) type connectors of the female type. Usingthe same type and gender of connectors 310, 312 thus minimizes microwavesignal reflection that would otherwise arise from gender conversionadapters.

Connectors 310, 312 comprise a material with high thermal conductivity(above a threshold) in the cryogenic temperature range. In anembodiment, connectors are formed using a metal that exhibits a RRR ofat least 100, and a thermal conductivity of greater than a 1 W/(cm*K) at4 Kelvin, threshold level of thermal conductivity. For example,connectors 310, 312 may be formed using gold, silver, copper, oraluminum. In an embodiment, connectors 310, 312 are single poleconnectors. For example, connectors 310, 312 may couple to respectivetransmission lines 304, 308 at a single contact point.

These examples of the types, genders, and material-combinations ofconnectors 310, 312 are not intended to be limiting. From thisdisclosure, those of ordinary skill in the art will be able to conceivemany other types, genders, and material-combinations of microwaveconnectors and the same are contemplated within the scope of theillustrative embodiments. For example, SubMiniature version P (SMP) typeconnectors, or many other types of microwave and radio frequencyconnectors, of the same gender, and of a suitable material or materialcombination, can be formed and used as connectors 310, 312 within thescope of the illustrative embodiments.

Housing 314 comprises a material with high thermal conductivity (above athreshold) in the cryogenic temperature range. For example, housing 314may be formed using gold, silver, or copper. Housing 314 couples tosubstrate 302. Housing 314 acts as a heat sink, transferring thermalenergy away from the substrate 302 and transmission lines 304, 306, 308,thereby minimizing noise in the transmission lines 304, 306, 308 fromthe thermal energy. In an embodiment, housing 314 comprises oxygen-freecopper material. In an embodiment, housing 314 comprises electrolyticcopper material.

With reference to FIG. 4, this figure depicts microwave simulationresults of the scattering parameters of an on-chip microwave filterexample. The on-chip microwave filter circuit, whose scatteringparameters are shown in FIG. 4, is based on the on-chip microwave filter300 exhibited in FIG. 3.

In FIG. 4, the graph 402 represents the transmission parameter S₁₂,while the graph 404 represents the reflection parameter S₁₁. Graph 402shows significant attenuation for transmitted signals above (e.g., 20 dBaround 7.5 GHz) and below (e.g., 20 dB around 3 GHz) the qubit signalrange (e.g., 3-5 GHz), while allowing the signals in the qubit signalrange to pass moderately filtered (i.e., filtered by about 5-7.5 dB).

With reference to FIG. 5, this figure depicts a flowchart of an examplemethod 500 for assembling an on-chip microwave filter. In block 502,transmission lines are deposited on a substrate. For example,transmission lines may be deposited using any conventional physical orchemical thin film deposition process, such as thermal evaporation,chemical vapor deposition, and sputtering. In an embodiment, at leasttwo transmission lines are deposited onto the substrate. In block 504,connectors are coupled to the transmission lines at respective ends ofthe substrate. For example, the connectors may be soldered to thetransmission lines. In block 506, a housing is coupled to the substrate.For example, the housing may be coupled to the substrate by fasteners,such as screws. In an embodiment, fasteners fabricated from a materialthat exhibits thermal conductivity above a threshold level of thermalconductivity couple the housing to the substrate.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., over, below,adjacent, etc.) are set forth between elements in the followingdescription and in the drawings, persons skilled in the art willrecognize that many of the positional relationships described herein areorientation-independent when the described functionality is maintainedeven though the orientation is changed. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present invention is not intended to be limiting inthis respect. Accordingly, a coupling of entities can refer to either adirect or an indirect coupling, and a positional relationship betweenentities can be a direct or indirect positional relationship. As anexample of an indirect positional relationship, references in thepresent description to forming layer “A” over layer “B” includesituations in which one or more intermediate layers (e.g., layer “C”) isbetween layer “A” and layer “B” as long as the relevant characteristicsand functionalities of layer “A” and layer “B” are not substantiallychanged by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “illustrative” is used herein to mean “serving asan example, instance or illustration.” Any embodiment or designdescribed herein as “illustrative” is not necessarily to be construed aspreferred or advantageous over other embodiments or designs. The terms“at least one” and “one or more” are understood to include any integernumber greater than or equal to one, i.e. one, two, three, four, etc.The terms “a plurality” are understood to include any integer numbergreater than or equal to two, i.e. two, three, four, five, etc. The term“connection” can include an indirect “connection” and a direct“connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. An on-chip microwave filter circuit comprising: asubstrate formed of a first material that exhibits at least a thresholdlevel of thermal conductivity, wherein the threshold level of thermalconductivity is achieved at a cryogenic temperature range in which aquantum computing circuit operates; and a dispersive componentconfigured to filter a plurality of frequencies in an input signal, thedispersive component comprising: a first transmission line disposed onthe substrate, the first transmission line being formed of a secondmaterial that exhibits at least a second threshold level of thermalconductivity, wherein the second threshold level of thermal conductivityis achieved at a cryogenic temperature range in which a quantumcomputing circuit operates; and a second transmission line disposed onthe substrate, the second transmission line being formed of the secondmaterial.
 2. The circuit of claim 1, further comprising: a connectorcoupled to the first transmission line, the connector being formed of athird material that exhibits at least a third threshold level of thermalconductivity, wherein the third threshold level of thermal conductivityis achieved at a cryogenic temperature range in which a quantumcomputing circuit operates.
 3. The circuit of claim 2, furthercomprising: a second connector coupled to the second transmission line,the second connector being formed of the third material.
 4. The circuitof claim 1, further comprising: a housing coupled to the substrate, thehousing being formed of a fourth material that exhibits at least afourth threshold level of thermal conductivity, wherein the fourththreshold level of thermal conductivity is achieved at a cryogenictemperature range in which a quantum computing circuit operates.
 5. Thecircuit of claim 1, the dispersive component further comprising: a thirdtransmission line disposed on the substrate, the third transmission linebeing formed of the second material.
 6. The circuit of claim 5, whereinthe third transmission line is disposed on the substrate between thefirst transmission line and the second transmission line.
 7. The circuitof claim 5, wherein the third transmission line is spaced apart on thesubstrate from the first transmission line.
 8. The circuit of claim 1,wherein the first transmission line has a thickness of about 10 nm to1000 nm.
 9. The circuit of claim 1, wherein the first transmission linehas a width of about 0.5 mm.
 10. The circuit of claim 2, wherein thesecond material and the third material are the same.
 11. A methodcomprising: forming a substrate, the substrate being formed of a firstmaterial that exhibits at least a threshold level of thermalconductivity, wherein the threshold level of thermal conductivity isachieved at a cryogenic temperature range in which a quantum computingcircuit operates; and forming an on-chip microwave filter on thesubstrate by assembling a circuit having two ports, the circuitcomprising: a dispersive component to filter a plurality of frequenciesin an input signal, the dispersive component comprising: a firsttransmission line deposited on the substrate, the first transmissionline being formed of a second material that exhibits at least a secondthreshold level of thermal conductivity, wherein the second thresholdlevel of thermal conductivity is achieved at a cryogenic temperaturerange in which a quantum computing circuit operates; and a secondtransmission line deposited on the substrate, the second transmissionline being formed of the second material.
 12. The method of claim 11,further comprising: forming a housing, the housing comprising: aclosable structure in which the circuit is positioned, the structurebeing formed of a third material that exhibits at least a thresholdlevel of thermal conductivity, wherein the threshold level of thermalconductivity is achieved at a cryogenic temperature range in which aquantum computing circuit operates.
 13. The method of claim 11, furthercomprising: coupling a first connector to the first transmission line,the first connector being formed of a fourth material that exhibits atleast a threshold level of thermal conductivity, wherein the thresholdlevel of thermal conductivity is achieved at a cryogenic temperaturerange in which a quantum computing circuit operates.
 14. The method ofclaim 13, further comprising: coupling a second connector to the secondtransmission line, the second connector being formed of the fourthmaterial.
 15. The method of claim 13, wherein the fourth material andthe second material are the same.
 16. The method of claim 11, whereinthe dispersive component further comprises: a third transmission linebeing formed of the second material.
 17. The method of claim 16, whereinthe third transmission line is disposed between the first transmissionline and the second transmission line on the substrate.
 18. The methodof claim 16, wherein the third transmission line is spaced apart on thesubstrate from the first transmission line.
 19. The method of claim 11,wherein the first transmission line has a thickness of about 10 nm to1000 nm.
 20. A fabrication system performing operations comprising:forming a substrate, the substrate being formed of a first material thatexhibits at least a threshold level of thermal conductivity, wherein thethreshold level of thermal conductivity is achieved at a cryogenictemperature range in which a quantum computing circuit operates; andforming an on-chip microwave filter on the substrate by assembling acircuit having two ports, the circuit comprising: a dispersive componentto filter a plurality of frequencies in an input signal, the dispersivecomponent comprising: a first transmission line deposited on thesubstrate, the first transmission line being formed of a second materialthat exhibits at least a second threshold level of thermal conductivity,wherein the second threshold level of thermal conductivity is achievedat a cryogenic temperature range in which a quantum computing circuitoperates; and a second transmission line deposited on the substrate, thesecond transmission line being formed of the second material.